JPH0216280B2 - - Google Patents

Description

Claim 1. Growth of mercury cadmium telluride crystals comprising the steps of forming a mercury cadmium telluride melt in a sealed ampoule and growing mercury cadmium telluride crystals from the melt by a vertical Bridziman process. In the method, mercury cadmium telluride crystals are grown at a rate of 0.1 to 2 mm/hour, and a periodic accelerated crucible rotation program is applied to the ampoule during the growth of the mercury cadmium telluride crystal, and this program A method for growing mercury cadmium telluride crystals, comprising the steps of accelerating the ampoule's rotational speed about a longitudinal axis and decelerating the ampoule's rotational speed.

2 The maximum ampoule rotation speed is 10 to 100 r.pm,
A method according to claim 1, wherein the ampoule rotation speed is accelerated and decelerated at 5 to 40 r.pm/sec.

3. The method according to claim 1 or 2, wherein the ampoule is rotated in one direction.

4 a step in which the program accelerates the ampoule rotational speed from zero to a predetermined rotational speed; and a step in which the ampoule is rotated at the predetermined rotational speed for a predetermined period of time;
A step of decelerating the rotational speed to zero, a step of accelerating the rotational speed in the opposite direction to a predetermined rotational speed, a step of rotating the ampoule at this predetermined rotational speed for a predetermined period of time, and a step of decelerating the ampoule rotational speed to zero. A method according to claim 1 or 2, comprising:

5. The method according to any one of claims 2 to 4, wherein the ampoule is stopped from rotating for a predetermined period between the step of reducing the rotational speed and the step of accelerating the rotational speed.

6. The method according to any one of claims 1 to 5, wherein the ampoule has a substantially flat bottom.

7. The method according to any one of claims 1 to 6, wherein the mercury cadmium telluride crystal has a composition represented by the formula CdxHg ₁ -xTe (0.4x0.15).

8. The method according to any one of claims 1 to 7, wherein the mercury cadmium telluride crystal is grown at a rate of 0.5 mm/hour.

Description The present invention provides a mercury cadmium telluride crystal comprising the steps of creating a mercury cadmium telluride melt in a sealed ampoule and growing the mercury cadmium telluride crystal from the melt by a vertical Bridziman process. It concerns how to grow.
Vertical Bridgeman process is understood to mean a process in which a closed ampoule containing a melt is slowly lowered through a vertically extending temperature gradient to initiate crystallization at the bottom of said ampoule.

In manufacturing semiconductor devices from ternary semiconductor compounds, a key issue is producing sufficient quantities of ternary compound crystals within a given composition range to achieve desired properties, such as mercury cadmium telluride for use in infrared detection. The goal is to provide materials with the desired electro-optical properties. The usefulness of this crystalline material also depends on the defects in the material.

Accelerated Crucible Rotation Technology (ACRT) is HJScheel
and Schulz-Dubois, “Journal.
of. crystal. Growth (Journal of
8 (1971) pp. 304-306, "Flux Growth of Large Crystals by
Accelerated Crucible−Rotation Technique”
It is proposed in ACRT has been used in a variety of crystal growth methods conducted in closed vessels, such as Czyochralski growth and solution-flux growth. The problem of high vapor pressure of the solutions used in growing crystals from hot solutions necessitates the use of closed containers, which precludes the use of conventional stirring techniques. Typically, the main improvement in using ACRT in such cases is to reduce nucleation, thereby increasing the size of the grown crystals compared to growth without agitation. The stirring action reduces the thickness of the diffusion boundary layer so that faster stable growth rates can be used. ACRT is known for example in U.S. Patent No.
It has been used to control oxygen content gradients in silicon crystals, as described in US Pat. No. 4,040,895.

FVWald and ROBell (Journal of Crystal Growth 30 (1974) No. 29-36
(page) describes a moving heater method for solution growth of CdTe from a solution in tellurium with ACRT applied during growth. The results obtained by this method are
We show that ACRT is not very effective (ACRT can only increase the growth rate at which inclusion-free crystals can be obtained due to two factors), which occurs under certain experimental conditions. This is because natural convection is already extremely strong. Furthermore, we found that the undesirable tendency of CdTe crystals grown by this method to twin and form stacking faults or low-angle grain boundaries still exists even when ACRT is used.

In _{the accompanying} drawings, _{FIG .}
FIG _. 2 is a graph plotting the mole fraction x of CdTe against the distance L from the tip of the ingot _, and FIG. 2 is a graph plotting the composition change in the radial direction against λ _e .

0.5 by the vertical Bridziman method from a melt with an initial composition of Cd _0.19 Hg _0.81 Te which was contained in an ampoule and rotated continuously at 2 r.pm during growth.
An ingot of mercury cadmium telluride grown at a rate of mm/hour has an axial composition (expressed as CdTe x mole %) as shown by curve A in Figure 1.
has. There is a compositional variation along the length of the ingot as cadmium telluride precipitates with respect to mercury telluride. When materials within a narrow composition range are required, 1
The yield of such material from a single ingot is small. In FIG. 2, curve C is a plot of the axial composition change Δx of mercury cadmium telluride in this ingot against the axial cut-on wavelength λ _e of the 300K ingot. Considering curves A and C together reveals the low yield of useful material grown by this method.

It is an object of the present invention to provide a method for growing mercury cadmium telluride crystals with excellent compositional uniformity.

The present invention uses mercury cadmium telluride crystals from 0.1 to
growing at a rate of 2 mm/hour, applying a periodic accelerating crucible rotation program to the ampoule during growth of the mercury cadmium telluride crystal, which program accelerates the ampoule rotation rate about the longitudinal axis of the ampoule; and a step of reducing the rotational speed of the ampoule.

To optimize the yield of uniform material produced,
The maximum ampoule rotation speed is 10 to 100 r.pm, and the acceleration and deceleration of the ampoule rotation speed is 5 to 40 r.p.
Preferably, it is m./sec. This ampoule can be rotated in one direction.

The accelerated crucible rotation program includes a step of accelerating the ampoule rotation speed from zero to a predetermined rotation speed;
A step of rotating the ampoule at this predetermined rotational speed for a predetermined period of time, a step of decelerating the rotational speed to zero, a step of accelerating the rotational speed in the opposite direction to a predetermined rotational speed, and a step of rotating the ampoule at this predetermined rotational speed. The ampoule rotation speed may be reduced to zero. Similar A without reversing the direction of rotation of the ampoule.
CRT programs can be used. The ampoule can be kept in a state where it stops rotating for a predetermined period between the step of reducing the rotational speed and the step of accelerating the rotational speed. The bottom of the ampoule can be, for example, conical, generally hemispherical or flat. The following table sets forth a preferred series of ACRT programs suitable for use with Bridgeman ampoules in the method of the invention.

[Table] The method of the present invention can be applied, for example, to the formula Cd _x Hg _1-x Te (however,
0.4x0.15) can be used to grow mercury cadmium telluride crystals. During the research leading to the present invention, the effect of the growth rate of mercury cadmium telluride crystals grown by the vertical Bridgeman method on the axial and radial composition of the grown material was investigated. The ampoules used in this experiment were rotated at a constant speed of 2 r.pm and different growth rates from 0.25 to 5 mm/h were used. It has been found that when slow growth rates are used, substantially uniform radial composition uniformity can be obtained, but large variations in axial composition occur. At fast growth rates, mercury telluride accumulated in front of the growth interface, while at slow growth rates there was mixing of the resulting liquid phase, which dispersed the mercury telluride-rich layer. If the mercury cadmium telluride melt is properly mixed so that equilibrium is always maintained throughout the melt, when the melt is cooled, the melt composition will be CdTe-HgTe.
The system changes along the liquidus line of the pseudo-binary phase diagram and the solid composition changes along the solidus line of said phase diagram (e.g. JLSchmit and CJSpeerschneider,
"Infrared. Physics"
Phys.) 8 (1968) 247. Significant segregation therefore always occurs under equilibrium growth conditions. If the mercury cadmium telluride melt is frozen from one end, as in the vertical Bridgeman process, equilibrium growth produces a continuously varying axial solid composition. We used the ACRT program to grow mercury cadmium telluride crystals by producing less melt mixing leading to the occurrence of equilibrium growth. We have found that it is possible to produce mercury cadmium telluride crystals with substantially constant axial and radial compositional uniformity over a significantly longer length of crystalline material at speeds than previously achievable.

Mercury cadmium telluride crystal _Cdx having desired electro-optical properties when using infrared radiation with wavelengths in the range 3-5 μm and in the range 8-13 μm
The composition of Hg _1-x Te is about 0.30 and about 0.21, respectively
has an x value of The yield from a particular ingot material with desired properties depends on the material's longitudinal composition distribution, radial composition distribution, and crystal quality.

During the research leading to the present invention, simulation studies were conducted to determine the flow present in ampoules used in the Bridgeman crystal growth method under the influence of ACRT. Three types of processes that can occur during accelerated/decelerated rotation are those that are faster than normal convection. transient form
quet flow (form) occurs exclusively during deceleration of the ampoule, and this type of flow is significant when relatively full ampoules are used. During rapid deceleration of the ampoule, the inner liquid rotates more rapidly than the outer liquid, which is closer to the wall.
As a result of the centrifugal force forcing the inner liquid towards the walls, very significant mixing of the melt occurs at the vessel walls, affecting the crystallinity and radial composition uniformity. Quet's flow reduces the natural nucleation that occurs on the vessel wall.

During acceleration or deceleration of liquid in an ampoule, spiral shearing of the liquid occurs, like a transient Couquette flow, because the rotation of the liquid closest to the container wall causes the internal liquid to Because it changes faster. liquid subject (the
Mixing in the bulk of the liquid is primarily due to Ekman flow and helical shear.

Ekman layer flow occurs in the liquid near a solid interface extending perpendicular to the axis of rotation of the ampoule. During acceleration of the ampoule, an Ekman layer forms just above the bottom of the ampoule, and within this Ekman layer there is a fast radial flow of liquid. This liquid is forced up along the side walls of the ampoule and returns relatively slowly through the main body of liquid before re-entering the Ekman layer. The conical shape of a typical Bridgeman ampoule creates a stagnation area that is unaffected by either acceleration or deceleration of the ampoule.

During acceleration of the ampoule, there is a strong downward center flow of liquid, a fast flow across the bottom and a spiral flow up the walls. In ampoules with a flat bottom, a stable flow section is created at the junction of the bottom and the side wall. During deceleration, there is a pine room-shaped flow of liquid from the bottom. Therefore one
During the ACRT cycle, the liquid undergoes significant vertical movement.

During the growth of mercury cadmium telluride, the Ekman flow tends to average out the radial segregation of HgTe. In the conventional Bridgeman process without ACRT, the HgTe tends to settle to the bottom of the concave liquid-solid interface.

When growing mercury cadmium telluride ingots using the method of the invention, the yield of material that can be used to make infrared detectors is
The yield was found to be about 10 times that obtained when using the Bridgeman method without ACRT. This is due to both the improvement in compositional uniformity and the improved crystalline quality of the material. Furthermore, it has been found that when using the method of the present invention, no twinning occurs and fewer low-angle grain boundaries are present than when using the prior art Bridziman method of growing mercury cadmium telluride crystals. Ivy.

Some examples of the present invention manufactured by the method of the present invention will be explained with reference to Examples and drawings. In the drawings, FIG. 3 is a schematic vertical cross-sectional view of a rocking furnace, FIG. 4 is a schematic cross-sectional side elevational view of the furnace used for growing mercury cadmium telluride crystals by the method of the present invention, and FIG. is the vertical temperature distribution diagram of the furnace shown in Fig. 4, and Fig. 6 shows the radial composition change of the mercury cadmium telluride ingot produced from the melt having the initial composition Cd _0.12 Hg _0.88 Te _. This is a graph plotted against.

Example 1 A conical base conventionally used in the vertical Bridgeman growth method, length 300 mm, inner diameter 13 ± 0.5 mm,
74.2986g in silica ampoule 1 with wall thickness 3±0.5mm
of mercury, 10.0285g cadmium and 59.9176g
of tellurium was added. The charge in this ampoule corresponded to the composition Cd _0.19 Hg _0.81 Te, but lacked 2 g of mercury. Ampoule 1 was evacuated and sealed, and a 10 mm diameter silica rod 2 was sealed to its bottom end. This sealed ampoule 1 was placed in a conventional rocking furnace 3 (shown in Figure 3). This rocking furnace 3 has an alumina furnace tube 4
The tube is fitted with alumina plugs 5, 6 at both ends.
These plugs served to reduce heat loss from both ends of the furnace. Heat ampoule 1 to 500℃ for 1 hour, then reduce the temperature of ampoule 1 to
The temperature rises to 825℃ over 24 hours, and the temperature rises to 2
Time was maintained. During the heat treatment, the furnace was rocked ±5° about the horizontal position at a rate of one cycle every two hours to form a uniform melt 7. Ampoule 1 was then cooled.

The cooled ampoule 1 was then transferred to a vertically arranged tubular electric furnace 8 (FIG. 4). The furnace 8 was equipped with a stainless steel tube 9 whose upper end was closed with a ceramic stopper 10. Ampoule 1 was placed inside furnace tube 9 and supported by silica rod 2, which was connected to a mechanical drive mechanism (not shown). This drive mechanism consisted of a variable low speed motor and a lifting/lowering mechanism. Initially, the entire charge in ampoule 1 was placed in an area in furnace 8 where the temperature was high enough to melt all the mercury cadmium telluride. Ampoule 1 is then subjected to a periodic ACRT program in which the ampoule
60r.pm from rest with increasing speed of 16r.pm/s
and maintain a constant speed of 60rpm for 8 seconds,
decelerates to standstill at a decreasing speed of 16r.pm/s,
Keep the rotation stopped for 1 second, then turn to 16r.p.
Accelerate in the opposite direction to 60r.pm with increasing speed of m./s,
Maintain constant speed of 60r.pm for 8 seconds, then 16r.pm.
It was decelerated to a stationary state at a decreasing rate of pm/sec and maintained at a stopped rotation for 1 second. This program was then repeated over the entire growth period. Ampoule 1 was lowered at a constant speed of 0.5 mm/hour as in the conventional vertical Bridgeman method. The molten charge 7 began to form an ingot 11 at the tip of the conical portion of the ampoule 1, and growth proceeded until all of the molten material had solidified. Ingot 11 is
A temperature gradient of 6°C/mm extending from 750°C to 700°C was passed. FIG. 5 shows the longitudinal temperature distribution of the vertical furnace 8, which is the classical Bridgeman temperature distribution.

Figure 1 shows the distance L from the conical tip of the ingot.
x (mercury cadmium telluride CdTe
(content) in the axial direction. Curve B relates to ingots produced by the method described above using a specific ACRT program; curve A relates to a control method that differs only in that the ampoule is rotated at a constant speed of 2 rpm over the entire growth period. It is. Figure 2 shows the radial composition change Δx
(CdTe mol %) is plotted against the cut-off wavelength λ _e at the center of the slice. Curve D relates to an ingot produced by the method described above using a specific ACRT program, and curve C relates to an ingot produced by the control method. From figures 1 and 2
Material grown using the ACRT program contains significant amounts of material useful for the 3-5 μm range, as well as useful material for the 7-8 μm range (at 20°C), i.e. 8-13 μm at 77K. It is clear that the ampoules contain significantly more material suitable for the purpose of the present invention than those produced by a similar vertical Bridgeman process in which the ampoules are rotated at a constant speed of 2 rpm. This is due to the significantly improved radial uniformity achieved by using the ACRT program.

Example 2 6.0551g of cadmium in ampoule 1,
77.2450g of mercury and 57.2813g of tellurium were charged. This charge corresponded to the composition Cd _0.12 Hg _0.88 Te but lacked 2 g of mercury. different ACRs
Ingots were made from this charge in a manner similar to that used in Example 1, except that the T.T. program was used. Identical acceleration and deceleration (16r.p.
m./s) and the same constant rotational speed (60r.pm)
was used. The rotation period at constant speed was 120 seconds in each case, and the period during which the ampoule stopped rotating was 30 seconds in each case.

Figure 6 shows the composition change in the radial direction λ _e (CdTe mol%)
is a graph plotted against the cutoff wavelength λ _e at the center of the slice. Curve E relates to ingots produced by the method described above using a specific ACRT program, and curve F uses the same starting composition but with the ampoule rotated at a constant speed of 2 rpm over the entire growth period. This relates to ingots produced by a control method.

Example 3 A method similar to that described in Example 2 using ACRT was used, except that the ampoule used had a flat bottom.

Curve G in Figure 6 shows the radial composition change Δx
(CdTe mol%) is plotted against λ _e .

By using ampoules with a flat bottom, the method of the present invention improves the radial composition of mercury cadmium telluride compared to material grown under similar conditions using ampoules with a conical bottom. It can be seen that the uniformity can be improved to some extent.

However, such improvement was achieved by using ampoules with a conical bottom and subjected to an ACRT program, as opposed to using similar ampoules rotated at a constant speed of 2 r.pm during the growth period. small compared to the improvement achieved.